[15.07] A Study of Seasonal & Short Period Variation of Water Ice Clouds in the Tharsis and Valles Marineris Regions of Mars with Mars Global Surveyor

The Mars Global Surveyor Mars Orbiter Camera was used to
obtain global maps of the Martian surface with equatorial
resolution of 7.5 km/pixel in two wavelength ranges: blue
(400-450 nm) and red (575-625 nm). The maps used were
acquired between March 15, 1999 (Ls = 110º) and July 31,
2001 (Ls = 205º), corresponding to approximately one and
a quarter martian years. Using the global maps, cloud area
(in km2) has been measured daily for water ice clouds
topographically corresponding to Olympus Mons, Ascraeus
Mons, Pavonis Mons, Arsia Mons, Alba Patera, the western
Valles Marineris canyon system, and for other small surface
features in the region. Seasonal trends in cloud activity
have been established for each of the five Tharsis
volcanoes. Olympus, Ascraeus, and Pavonis Mons show cloud
activity from about Ls = 0º-220º with a peak in cloud
area near Ls = 100º. One of our most interesting
observational results is that Alba Patera shows a double
peaked feature in the cloud area with peaks at Ls = 60º
and 140º and a minimum near Ls = 100º. Arsia Mons shows
nearly continuous cloud activity. A quasi-periodicity of
2.10-2.40 sols has been found in all five areas of the
Tharsis region examined. It is most likely the combined
result of the diurnal cloud variability during the afternoon
hours and the motion of the spacecraft’s orbit. In addition,
optical depth has been measured for 154 of the observed
clouds. In general, the optical depth of clouds in the first
martian year of our observations is smaller than that of the
second year, except for Alba Patera, where the optical
depths are similar. This work was supported by a Mars
Participation Scientist grant and by a MGS Team Science Data
Analysis grant, both from Jet Propulsion Laboratory.

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